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D. A. Frail (NRAO), E. Berger (Caltech), S. R. Kulkarni (Caltech)
We present Very Large Array (VLA) radio observations of GRB980703, from 1.43 to 8.46 GHz for the period of 450 to 1000 days after the burst. These radio data clearly indicate that there is a constant flux source at the position of GRB980703 with a flux density of approximately 70 microJy at 1.43 GHz, and a spectral index of approximately 0.32. We find that no afterglow model can account for this emission, and that the measured flux density at the three radio frequencies exceeds the afterglow contribution by 1-2 orders of magnitude. We therefore interpret this emission as coming from the host galaxy of GRB980703. This is the first example of a gamma-ray burst (GRB) host being detected at radio wavelengths, and this set of observations has unique implications for the star-formation rate (SFR) in the host and for GRB progenitor models. We find that the emission can be explained as a result of a SFR of massive stars (M>5 Modot) of 60 M\odot/yr, which gives a total SFR of 300 M\odot/yr using the Salpeter IMF. In addition, the radio measurements show that the offset between the burst and the host is -0.04± 0.02 arcsec in RA, and that the size of the host as traced by the radio emission is less than 2.3 kpc. These facts point to the association of GRB980703 with the region of maximum star-formation in the host, as expected if GRBs are associated with the collapse of massive stars. Finally, we show that the estimated SFR is consistent with upper limits at 350 GHz obtained with SCUBA.
We acknowledge support by NSF grant AST-9803157 and NASA grant NAS5-26555. The National Radio Astronomy Observatory (NRAO) is a facility of the National Science Foundation operated under cooperative agreement by Associated Universities, Inc. NRAO operates the VLA.